In the typical disc drives using a stepper motor, one must develop an algorithm for controlling the stepper motor to move the head from one track to another selected track that may be 100 or more tracks away. In the process of doing this, the motor is accelerated to very high speeds and then rapidly decelerated and stopped. This rapid acceleration and deceleration is implemented in part by putting the time between each step pulse in a lookup table accessible by the motor controlling microprocessor. These time periods are critical to the drive performance. A 1 microsecond shift in those time periods can throw the time periods off, so that the settling of the head relative to the tracks on the disc drive is no longer optimum.
Since the steppe3r motor in a hard disc drive will be exposed to a specific broad, prespecified thermal range (4.degree. C. to 70.degree. C.), the winding resistance of the stepper motor will change due to the varying temperature. This change is predictable and repeatable. Typically, the motor resistance (Rm) will change about 1.393%/ C. An equation for the motor resistance at any temperature can be expressed as the following: EQU R.sub.m =R.sub.o +R.sub.o T.sub.cm (T-T.sub.0) (1)
where
R.sub.o =Nominal resistance at ambient room temperature
T.sub.cm =Thermal resistive coefficient (0.393%/.degree. C. for copper)
T.sub.0 =Ambient room temperature of 25.degree. C.
T=operating temperature
It is detrimental to motor performance to have this increase in resistance at higher temperatures, since it reduces motor torque (from that at lower temperatures) and alters the L/R time constant of the system. The alteration of the L/R time constant is particularly harmful because the motor step algorithm that incorporates the lookup table described above cannot be retuned to account for this change in the electrical system.
The alteration of the current will result in a torque change in the motor. This change in torque can be minimized in part by tuning the motor voltage as is done by most disc drive manufacturers; but to a certain extent, the proper tuning of the motor is overcome by this change in current, since the change in motor voltage accomplished by tuning can partially change the current to compensate for the resistance changes, but the correction cannot be as complete and accurate as desired. That is, the L/R time constant will not change as the voltage applied to the phase is changed, and therefore, tuning by modification of the applied voltage will not accurately compensate for this alteration in the system. Thus, the potential alteration of the L/R time constant with changes in temperature must be accounted for in some other way in order to maintain the high degree of accuracy required in a disc drive.